1. Field of the Invention
The present invention relates to synchronous reluctance motors that are used for, e.g., electric power steering systems.
2. Description of the Related Art
Reluctance motors are known which rotate a rotor by using only reluctance torque generated by a change in electromagnetic energy with respect to the position. The reluctance motors include switched reluctance motors (SRMs) and synchronous reluctance motors (SynRMs). In the switched reluctance motors, a stator and a motor have a magnetic saliency. In the synchronous reluctance motors, a stator has a structure similar to that of a stator of a brushless motor.
In the synchronous reluctance motors, only the rotor has a magnetic saliency out of the stator and the rotor. In the synchronous reluctance motors, there are a salient direction in which magnetic flux tends to flow (hereinafter referred to as the “d-axis direction”) and a non-salient direction in which the magnetic flux is less likely to flow (hereinafter referred to as the “q-axis direction”) due to the magnetic saliency of the rotor. Accordingly, reluctance torque is generated due to the difference between inductance in the d-axis direction (hereinafter referred to as the “d-axis inductance”) and inductance in the q-axis direction (hereinafter referred to as the “q-axis inductance”), and the rotor is rotated by the reluctance torque.
The synchronous reluctance motors do not use permanent magnets and rotate the rotor by using only the reluctance torque. The synchronous reluctance motors are therefore disadvantageous in that their output torque is smaller than that of motors using permanent magnets. It is desired to increase the output torque of the synchronous reluctance motors as much as possible.